fetch dataset optionally from db to handle gracefully

[dinmukhamedm]

[!IMPORTANT] Improve dataset fetching by returning 404 for non-existent datasets in semantic_search, upload_datapoint_file, and index_dataset.

• Behavior:
  • semantic_search in semantic_search.rs now checks if a dataset exists and returns 404 if not found.
  • upload_datapoint_file and index_dataset in routes/datasets.rs now return 404 if dataset is not found.
• Database:
  • get_dataset in datasets.rs now returns Option<Dataset> instead of throwing an error if not found.

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